The United Nations humanitarian chief has warned that unless the Saudi-led military coalition lifts its blockade on Yemen the war-torn nation will face "the largest famine the world has seen for many decades, with millions of victims".

Mark Lowcock told reporters after briefing the UN Security Council behind closed doors yesterday that there must be an immediate resumption of regular air flights to the cities of Aden and Sanaa by the UN and its humanitarian partners.

He said there also has to be immediate access to all ports, especially for food, fuel, medicine and other essential supplies.

The coalition tightened its blockade in Yemen this week after a ballistic missile fired by Houthi rebels was intercepted near the Saudi capital - an act condemned by Lowcock and the Security Council.

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The UN said aid agencies were given no prior notice of the Saudi decision to shut down all land, air and seaports in Yemen.

Lowcock said he told the council that "there will be famine in Yemen" unless five steps are taken immediately. "It will not be like the famine that we saw in South Sudan earlier in the year where tens of thousands of people were affected," he said.

"It will not be like the famine which cost 250,000 people their lives in Somalia in 2011. It will be the largest famine the world has seen for many decades, with millions of victims."

He said the five steps are:

● Resumption of air services.

● Assurances flights will not be disrupted.

● Opening of all ports to humanitarian and commercial vessels, especially those with critical supplies.

● Agreement to keep a UN World Food Programme ship in waters off Aden.

● Halt to interference with all vessels that have passed inspection by the UN Verification and Inspection Mechanism "so that they can proceed to port as rapidly as possible".

David Beasley, executive director of the World Food Program, said this week that out of Yemen's population of 27 million to 28 million people, 19 million to 20 million "do not know where they're going to get their next meal".